note about content providers

doc/developer/repoproviders.rst

@@ -38,7 +38,8 @@ It is possible to add new repository providers to BinderHub, allowing
 a BinderHub deployment to fetch repositories from new locations
 on the web. Doing so involves defining your own RepoProvider sub-class
 and modifying a set of methods/attributes to interface with the online
-provider to which you are providing access.
+provider to which you are providing access. It also often involves
+`building a new repo2docker content provider <https://github.com/jupyter/repo2docker/tree/master/repo2docker/contentproviders>`_.
 
 In order to extend the supported repository providers,
 follow these instructions. We'll provide example links for each step to a